In the 5th Generation (5G) wireless systems in standardization, beamforming and Multiple Input Multiple Output (MIMO) transmission will be central technologies. Increasing capacity requirements is driving this development where increasing amounts of MIMO transmission is introduced in existing frequency bands. However, this will soon become insufficient, thereby requiring migration into spectrum at higher carrier frequencies, starting at 3.5-5 GHz, continuing to the soon available 28 GHz and 39 GHz bands and beyond, towards 60-100 GHz. For these higher bands, beamforming with massive antenna arrays, in the end with hundreds of elements, will be needed to compensate for the worsening radio propagation conditions, caused by the decreasing area of single dipole antenna elements. At these higher frequencies cost, size, power and interface constraints may prevent the use of individual digital data streams towards individual antenna elements, thereby most probably restricting solutions to so called analogue beamforming, or possibly hybrid or constrained beamforming with a few signal layers that can be used for multi-user access, over time and frequency.
At the same time the dominating multi-user access technology for 5G is expected to become some variant of Orthogonal Frequency Division Multiple access (OFDM). This access is associated with a resource grid, divided in time and frequency. A division in frequency is defined by sub-carriers and a division in time is defined by OFDM symbols. When multi layered (MIMO) transmission is used, there is one overlaid resource grid per layer, separated by spatial pre-coding.
It is noted that with analogue, hybrid or digitally constrained beamforming there is only one digital signal chain that may use pre-coding, per port. The advantage is that the signals to the many more antenna elements are then split and distributed by other analogue or digital signals towards the antenna elements, i.e. a port expansion is performed. As a consequence the analogue-to-digital/digital-to-analogue (AD/DA) conversion needs are minimized in case that would limit analogue beamforming products. In the same way, the interface requirements between base band where the MIMO pre-coding takes place and the radio Application Specific Integrated Circuit (ASIC) is also minimized. This is important since the interface capacity may constitute the bottleneck for the product. The consequence is that pure high gain analogue or interface constrained digital beamforming is restricted to scheduling of single or very few UEs, per symbol time.
This a problem since different applications have very different needs when it comes to the transmission rate. Voice traffic e.g. require <1 kbit/20 ms, whereas video download has a more or less unlimited bit rate need. Therefore, to avoid wasted capacity it is essential that the number of sub-carriers and the symbol time allows a fine enough granularity in terms of the total number of bits when combined.
Unfortunately, the Third Generation Partnership Project (3GPP) 5G standardization seems to become based partly on a re-scaling of the 4th Generation (4G) Long-Term Evolution (LTE) resource grid, which has a maximum bit rate per OFDM symbol of roughly 100000 bits, which is about 100 times larger than what is needed for voice. Later, standard developments introduce slots and mini-slots in time, however the granularity is still too coarse. In the case with pure scaling of the resource grid much more than 90% of the available resources would be wasted when applying analogue, hybrid or digitally constrained beam forming.
As a conclusion, the 3GPP 5G time-frequency granularity is adapted to digital beamforming and is too coarse to support low data rate users with good spectral efficiency, when analogue, hybrid or constrained beamforming is used at high carrier frequencies. This cannot match the granularity with e.g. WIFI present standardization ideas.